Sony WF-1000XM3 Charging Case Compatible Battery 3.7V 800mAh

Sony WF-1000XM3 Charging Case — 3.7V Li-ion Replacement Battery (1588-0911)

This is a 3.7V, 800mAh Li-ion replacement for the internal battery inside the Sony WF-1000XM3 charging case. It also fits the WF-SP900 charging case. When the case battery degrades, the case stops holding enough charge to top up the earbuds away from a wall outlet — this cell restores that function.

• WF-1000XM3 and WF-SP900 case compatibility: Both cases use the same 1588-0911 cell on a shared 3.7V rail with the same physical footprint and connector orientation. The BMS in each case communicates with the cell over the same two-pin logic line, so one replacement covers both.
• Bench tested on actual hardware: We ran the cell through full charge and discharge cycles on a WF-1000XM3 case. The BMS accepted the new cell without errors, and the case delivered consistent charge passes to the earbuds across repeated cycles.
• First charge after installation: Place the case on a USB-C charger and run it to 100% before putting earbuds in. The case BMS needs one full cycle to log the new cell's capacity — skip this and the LED charge indicator may read inaccurately for the first week of use.

Why the WF-1000XM3 case shows full charge but the earbuds go flat anyway

The case has no direct voltage readout — it estimates remaining capacity from the cell's state of charge curve. When the original cell ages, its voltage curve flattens and the BMS misreads remaining capacity by a wide margin. A depleted cell sitting at storage voltage will report 100% right after a short partial charge, then drop off fast under earbud load. Replacing the cell resets the curve the BMS tracks against.

Case LED stays solid white but earbuds are not charging inside

A solid white LED means the case thinks it has charge available, but a severely degraded cell can hold just enough voltage to trigger the LED without sustaining the current needed to push charge into the earbuds. The earbuds draw a small but steady current when seated — if the case cell voltage sags below 3.4V under that load, the charging circuit shuts off silently. Fit the new cell and run one full USB-C charge cycle before testing earbud charging. Check that the earbuds seat correctly in the contacts before ruling out a cell issue.